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Molecular and Morphological Evidence for Interspecific Hybridization Between Cronartium ribicola and C. comandrae on Pinus flexilis in Southwestern Alberta

In May 2003, a survey was conducted in southwestern Alberta, east of the Rocky Mountains, to determine the extent of the spread and genetic diversity of white pine blister rust, which is caused by Cronartium ribicola J.C. Fisch. Aeciospores were sampled from white pine blister rust cankers in three infected limber pine (Pinus flexilis James) stands separated from one another by 100 to 215 km. DNA genotypes were determined for 12 codominant PCR-SSCP (polymerase chain reaction-single strand conformation polymorphism) loci representing genes derived from an EST library. At each site sampled, some aecia displayed DNA genotypes that were heterozygous at all loci and possessed novel alleles (GenBank Accession Nos. DQ009533-DQ009611). At Waterton Lakes, Kananaskis County, and Porcupine Hills, 29%, 11%, and 3% of sampled aecia and 38%, 33%, and 10% of sampled trees, respectively, possessed these unusual profiles. In May 2004, similar genetic profiles were found at two of these sites, Waterton Lakes and Kananaskis County, at 17 and 25% of sampled aecia (25% of sampled trees). In each of these aecia, genotyping and sequence analysis revealed this pattern was due to the presence of one C. ribicola and one C. comandrae Peck. allele at each of the 12 loci. Scanning electron microscopy (SEM) revealed aeciospore morphology that was intermediate between C. ribicola and C. comandrae. Aeciospores were longer (16 to 20 × 25 to 40 μm) than the expected range for C. ribicola (18 to 20 × 22 to 31 μm) (3). They were also fusiform, obovoid or short-to-long ellipsoid, but not pyriform-acuminate as in C. comandrae, and without a true conspicuous smooth spot as in C. ribicola. This provides evidence for interspecific hybridization between C. ribicola and C. comandrae, the causal agent of comandra blister rust. We hypothesize that the presence of nearby C. comandrae-infected lodgepole pine (P. contorta Dougl.) could have led to spermatization of C. ribicola receptive hyphae by C. comandrae pycniospores, resulting in the formation of hybrid aecia. An important question is whether these hybrids have a different host range that could potentially extend its geographic range in areas where the telial host, Ribes spp. L., is not abundant. The hybrid rust Melampsora × columbiana Newcombe was shown to exhibit virulence against certain hybrid poplar clones that had previously been reported as resistant against both parental rusts (M. medusae Thuem. and M. occidentalis Jacks) and abundant pathogenic variation has been observed (2). Furthermore, the ability to colonize unexpected hosts could provide fitness advantages over parental species, as was observed in Phytophthora spp. pathogenic on alder (1). Host range and virulence assays should be conducted to assess the potential impact of this hybrid.

References: (1) C. M. Brasier et al. Proc. Natl. Acad. Sci. USA 96:5878, 1999. (2) G. Newcombe et al. Phytopathology 91:981, 2001. (3) W. G. Ziller. The Tree Rusts of Western Canada. Can. For Serv. No. 1329. Pacific Forestry Center, Victoria, BC, 1974.